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Abstract:

ABSTRACT: The main objective of this research is to develop a simple and cost effective separation method that captures carbon dioxide from power plant flue gas, as a pure stream that can be stored using regenerable dolomite (calcium magnesium carbonate) as the sorbent. The developed dolomite sorbent was evaluated for carbon dioxide capture capacity using muti-cycle tests of cyclical carbonation/calcination experiments in the thermogravimetric analyzer (TGA) model SDT 600. The variables controlled in the experiment were weight of calcium oxide and sintering time of the sample. The dolomite materials investigated were from two sources Alfa Aesar and Specialty Minerals. The prepared sorbent, after conditioning, is in the oxide form and can adsorb CO2 to form the carbonate and be regenerated back to the oxide. The results showed that the dolomite sorbent developed can be used for reversible CO2 capture. The data from 8 multi-cycle TGA experiments show that the reversible capacity reduced in the first few cycles; however it stabilized to an average value of 34 percent after an average of 10 cycles and an average conditioning time of 15 hours. Data from two multi-cycle TGA experiments show that the dolomite sorbent is capable of an average stabilized conversion of 65% in an average of 13 cycles at a conditioning time of 87 hours.

Thesis:

Thesis (M.S.Ch.)--University of South Florida, 2010.

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by Drupatie Latchman.

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ACKNOWLEDGEMENTS This research was conducted with the suppor t of the Clean Energy Research Center (CERC) at the University of South Florid a, Tampa Electric Company, and the Florida Energy Systems Consortium (FESC).

vi CARBON DIOXIDE CAPTURE FROM FO SSIL FUEL POWER PLANTS USING DOLOMITE Drupatie Latchman ABSTRACT The main objective of this research is to develop a simple and cost effective separation method that captures CO2 from power plant flue gas, as a pure stream that can be stored using regenerable dolomite (calci um magnesium carbonate) as the sorbent. The developed dolomite sorbent was evaluated for CO2 capture capacity using muti-cycle tests of cyclical carbonation/ calcination experiments in th e thermogravimetric analyzer (TGA) model SDT 600. The variab les controlled in the expe riment were weight of calcium oxide and sintering time of the sample The dolomite materials investigated were from two sources Alfa Aesar and Specialt y Minerals. The prepared sorbent, after conditioning, is in the oxid e form and can adsorb CO2 to form the carbonate and be regenerated back to the oxide. The results showed that the dolomite sorben t developed can be used for reversible CO2 capture. The data from 8 multi-cycle TGA experiments show that the reversible capacity reduced in the first few cycles; however it stabilized to an average value of 34% after an average of 10 cycles and an aver age conditioning time of 15 hours. Data from two multi-cycle TGA experiments show that the dolomite sorbent is capable of an

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vii average stabilized conversion of 65% in an av erage of 13 cycles at a conditioning time of 87 hours.

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1 CHAPTER 1. INTRODUCTION 1.1 Carbon Dioxide Emissions Carbon dioxide (CO2) is a clear colorless gas that is present everywhere in the atmosphere. It is one of the most important gases essential to life because plants use CO2 to produce food and in the process oxygen is released. However, CO2 has been implicated as major cause of climate change because it is a known greenhouse gas and its atmospheric concentration has significantly in creased over the last four decades (Figure 1). Most of this increase has been attribut ed to global industrializ ation. The United States and China alone account for more than 40% of the worlds CO2 emissions (Figure 2) [1]. Climate change is expected to have pr ofound effects on the environment and on human socioeconomic systems. This has prompted the United States Environmental Protection Agency (USEPA), for example, to classify CO2 as a pollutant in 2009, which would trigger measures to reduce the quantity released into the atmosphere [2]. There are many approaches to reducing CO2 emissions from industrial sources, particularly from power genera tion facilities. These include switching to alternative fuels (low carbon or carbon-free fuels) sources a nd improving process efficiency. However, many trends indicate that fossil fuels, especia lly coal and natural gas, will continue to serve as the predominant energy source for decades and therefore carbon capture and storage is an important a pproach currently being inve stigated. There are several

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2 technologies available to capture CO2, but they are currently costly to install and operate. They can be installed in preor post-combustion processes. All capture technologies are based on trapping the gas in a suitable me dium, solid sorbent or liquid absorber. CO2 emissions can to be addressed via: capt ure and storage of future man-made carbon dioxide emission, reducing the existing quanti ties of carbon dioxide from the atmosphere, and restoring the carboncycl e to pre-industrial era. Figure 1. Carbon Dioxide Concentration [3]

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3 Capture and storage technol ogy of future man-made carbon dioxide emission has made some advancement; however, no single te chnology exists today that addresses the pollution problem. Figure 2. World Carbon Dioxide Levels [4] According to the Intergovernmental Panel on Climate Change (IPCC) Special Report on CO2 the following is a profile of CO2 emissions by process or industrial activity of worldwide large stationary CO2 sources with emissions of more than 0.1 million tons of CO2 (MtCO2) per year [5].

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4 Table 1. Global CO2 sources (taken from [5]). Process Number of sources Emissions (MtCO2 per yr) Fossil fuels Power 4,942 10,539 Cement production Refineries 1,175 932 Iron and steel industry 269 646 Petrochemical industry 470 379 Oil and gas processing Not available 50 Other sources 90 33 Biomass Bioethanol and bioenergy 303 91 Total 7,887 1 ,466 The above table shows that electricity production worldwide is the largest contributor to CO2 emissions. It is clear that devel oping technologies to capture and store CO2 from these large point sources will have the biggest impact on CO2 emissions. Metz [5] also states that ot her options to control CO2 emissions are energy efficiency improvements and implementation, use of lower carbon fuels, nuclear power, renewable energy sources, enhancement to biological sinks, and reduction of other greenhouse gases. Currently, CO2 is used in many processes, but CO2 is often produced for use in these processes instead of using the existing CO2 supply in the atmosphere because of purity requirements. Acco rding to Edwards[6], CO2 is used in the manufacturing of products, such as chemicals, fertilizers, carbonated beverages, f ood preservatives, fire extinguishers, and it is even injected into oil and petroleum wells to improve production and aid recovery. Edwards [6] goes on to state that the rate at which CO2 is consumed is less than the quantity currently emitted, therefore an impact will not be seen, but every

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5 molecule of CO2 recycled and removed from the atmosphere, along with the elimination of new CO2 produced from fresh feedstock, will go a long way in helping to reduce CO2 levels. Fossil fuel combustors are the major contributors of CO2 emitted into the atmosphere (Figure 3). Power plants are the la rgest subset of fossil fuel combustors that are CO2 polluters. Fossil fuel provides approxima tely 85% of the energy the world needs [7]. Steps can be taken to control CO2 levels by capture and storage of emissions, using alternative sources of energy and using energy efficiently. Le e et. al.[8], developed a CO2 capture method using immobilized calcium oxide (CaO) on yttria and alumina substrates. The method showed with a 23 weight% samp le the conversion was 75% and with a 55 weight% sample the conversion was 62% over 13 and 10 cycles respectively. This research is a continuation of the Lee et. al .[8] research; however, dolomite according to Silaban et. al. [9] dolomite would have a hi gher capacity without in creasing the cost.

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6 Figure 3. Carbon Dioxide Emission Sources [4] 1.2 Research Objectives and Scope The overall objective of this project was to develop an inexpensive and efficient sorbent to capture CO2 and the design a reactor that uses the sorbent to capture CO2 emissions from power plants. The central hypothesis of the project was that dolomite would be a more efficient sorbent for the capture of CO2 than pure calcium carbonate in the given configuration because the magnesi um carbonate decomposition in the dolomite would increase the surface area after sint ering, which would increase the capacity to adsorb CO2. The rationale for this project is that the development of an effective sorbent for CO2 will allow scientists and engineers to de sign systems capable of capturing the gas and thereby contribute to the overall reduc tion of greenhouse gas emissions. The main focus of the research was the capture of CO2 emissions from power plants. However, the results of this project may be applicable for CO2 capture from other processes.

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7 The overall research objective was acco mplished through the following plan. The first objective was to develop a sorbent usi ng a method developed by Lee et. al. [10] to impregnate dolomite unto a ceramic fabric to capture CO2. It involves impregnating a ceramic fabric (alumina or yttria) with pure calcium carbonate. However, dolomite was used in this work instead of calcium carbonate. It is hypothesized that the inert magnesium in dolomite increases the surface area of the material during sintering, and thus the capacity to adsorb more gas. Th is occurs because magnesium carbonate has a lower melting point than calcium carbonate a nd, when the sorbent is sintered during preparation, the magnesium particles decompos e and leave pores in the sorbent so that the CO2 molecules can reach the in ternal calcium oxide partic les and react with them, instead of just the surfac e calcium oxide particles. The second objective was to quantify the conversion, determine the regeneration time needed, and evaluate the cyclic performance of the sorbent. CO2 molecules from flue gas react with the calcium oxide in the sorbent to form calcium carbonate. Heat is then used in the regeneration process to drive the CO2 molecules from the sorbent leaving a pure stream of CO2 that can be captured and reused or stored.

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8 CHAPTER 2. CARBON DIOXIDE CAPTURE TECHNOLOGIES The development of new and innovative te chnologies is critica l in resolution of the carbon dioxide pollution problem. Power plan ts have a huge role in curbing carbon dioxide emissions as they are the largest ca rbon dioxide emitters. The captured carbon can then be transported to an injection site for long-term storage in geologic formations for example. These technologies are currently being researched with some technologies in the early stages of resear ch and development [11]. There are several ways to capture CO2, but it must first be separated from other combustion gases [5]. The following are the three main capture technologies from power plants: 1. Pre-combustion 2. Post-combustion 3. Oxyfuel combustion Pre-combustion systems developed to capture CO2 are designed for Integrated Gasification Combine Cycle (IGCC) units [5]. Due to the nature of this process, it is easiest to remove pollutants prior to combusti on. The fossil fuel is reacted with steam and air /oxygen at very high temperat ures to produce a gaseous fuel that can be combusted in the turbine. This gaseous mixture is synthe tic gas or syngas consisting of carbon monoxide (CO), hydrogen (H2), methane (CH4), and water vapor (H2O). The CO can

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9 then undergo a water-gas shift reacti on, which will convert the CO to CO2 and H2. The CO2 is then captured in the reactor usi ng carbon capture techno logy. Therefore, CO2, like sulfur dioxide (SO2) and other pollutants, is removed before the syngas is fired in the turbine, preventing these pollutants from being emitted into the atmosphere. The benefit of pre-combustion CO2 capture is the ability to manipul ate the concentration and pressure of carbon dioxide prior to burning of the fuel This leads to a re duction in the capture equipment size and cost. The syngas can then be treated to remove pollutants and contaminants, leaving a clean, efficient fuel. For IGCC plants water-gas shift reactions equipment would need to be installed, for existing a nd new units, to produce a CO2 stream that can be separated and captured. Existing solvents and captu re technology can effectively remove CO2 because the high pressure syngas stream has high CO2 content [12]. Post-combustion CO2 capture system, on the other hand, removes CO2 after the combustion process. According to Metz et. al. [5], a liquid organic solvent such as monoethanolamine (MEA) would be used to capture the CO2 in the flue gas. This means of CO2 capture is more challenging as the concentration of CO2 is very low and there are other pollutants present in the flue gas, which makes separation very costly and difficult [11]. In the case of oxyfuel combustion systems, oxygen is used for combustion of the fossil fuel and it produces mostly water vapor and CO2 [5]. The high concentration of CO2 (greater than 80% by volume) can be cap tured after the water vapor is separated by cooling and compressing the flue gas and ot her pollutants are removed. This method of CO2 capture is very cost prohi bitive as the equipment needed for air separation and the

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10 production of oxygen is very expensive. Each of the three CO2 capture categories discussed above can utili ze any of the three CO2 separation technologies or a mixture of CO2 separation technologies to attain an efficient capture system. The three CO2 separation technologies are: solvents, membranes, and sorbents. However, chemical looping combustion, oxy-combustion, and cryogenic separation can also be used in the fight to reduce CO2 emissions and are briefly described. 2.1 Solvents The process of solvent absorption uses the reversible nature of the chemicals to remove the CO2 from the flue gas stream either p hysically or chemically [13, 14]. The solvent can then be regenerated by changi ng the operating conditions, that is, the temperature or pressure in th e system [14]. Both physical a nd chemical solvents can be used to capture CO2. Physical solvents remove the carbon dioxide selectively by absorbing the gas without any chemical interaction, usually or ganic liquids [11, 14]. Selexol and Rectisol, KS-2, and propylene carbonate are examples of physical solvents [15]. Here the CO2 partial pressure is proportional to the capacity of the absorb ing capability of the physical solvents according to Henrys law [11, 14]. Researchers are working to improve physical solvents by finding different and better solven ts, manipulating pressure, temperature and other conditions of both the flue gas and so lvent stream, and improving the selectivity of the solvent to CO2. Physical solvents use less energy fo r regeneration, but, they work best at low temperatures [11].

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11 Chemical solvents, on the ot her hand, facilitate a chem ical reaction to strip or extract the carbon dioxide from the flue gas stream and are usually aqueous solutions [13, 14]. MEA is example of a chemical solvent a nd is currently the mo st matured technology used to capture CO2 [15, 16]. Scholes et al., [12] al so investigated the use of hot potassium carbonate solvent system. MEA and amine solvents have a high removal percentage, but they use a lot of energy to regenerate and also decrease the electric output of the unit by about 15 to 60 percent depe nding on the type of ge nerating unit [13, 17]. Amine solvents degrade quickly and cannot be reused. They are also corrosive in nature, which affect the construction ma terial of equipment [12]. Applications of solvents for high pres sure and high temperature are being researched as part of the DECARBit projec t [17]. Technological advancement to amine systems are currently being developed by Fluor Mitsubishi Heavy I ndustries (MHI), and Cansolv Technologies [11]. Modifications a nd improvements to amine systems aim to reduce pressure drop, increase contacting, incr ease heat integration to reduce energy requirements, reduce corrosion, and impr ove regeneration procedures [11]. Ammonia wet scrubbing can also be used as a solvent to capture CO2 and it is similar to amine systems, but has lower energy requirements, higher CO2 capture capacity, lack of degradation, is tolerant to oxygen in the flue gas, low cost, and potential for regeneration at high pressure [11, 18]. Ye h et al. [18] compared aqueous ammonia solution to MEA. The study showed that the energy required for regeneration using aqueous ammonia instead of amine could be reduced by approximately 62% [18]. However, ammonia has a higher volatility than MEA, which is one of the main concerns about this solvent, as the flue gas must be cooled to improve absorption [11]. Figueroa et

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12 al., [11] also states that other technical i ssues that must be ove rcome to make this technology more attractive than using amine. Ionic liquids are liquids that are mostly made up of ions instead of molecules [11, 19]. This solvent can dissolve gaseous CO2 and is not lost into the gas stream [19]. Therefore, the flue gas does not have to be cooled before the solvent can come into contact with the flue gas, however the absorption capability of CO2 needs improvement [11]. Olivier-Bourbigou et al., [ 20] discusses and reviews ioni c liquids, their applications, and their properties in detail. 2.2 Membranes Membranes are semi-permeable materials that selectively allow molecules to pass through them [21]. Membrane separation can be a combination of adsorption and absorption. Polymer-based membranes are show ing promising results in the fight to lower the energy requirement and cost of carbon separation and capture. According to researchers at DOEs Los Alamos Nationa l Laboratory (LANL), a polybenzimidazole (PBI) membrane has shown signs that it can be durable in coal fire d power plants [11]. Researchers at the NETL in collaboration with the University of Notre Dame are working on a liquid membrane, which will selectively remove the CO2 molecules [11]. The membrane is made of an advanced poly mer substrate and an ionic liquid [11]. Another concept under development is the use of an inorganic membrane. Researchers are developing a micro-porous me mbrane to allow for the separation of CO2 from the flue gas [11, 22]. This modification allows the strong inte ractions between the permeating CO2 molecules and the amine functional membrane pores [11]. While in the

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13 New Mexico Institute of Mining and T echnology, zeolite membranes are being developed. Zeolites are micro-porous structur es of aluminosilicate minerals [11, 23]. Enzyme based membranes are also being inve stigated. Carbonic anhydrase is an enzyme that is contained in a hollow fiber [11]. Demonstrations show 90% CO2 capture followed by regeneration at ambient conditions. Limitations include membrane boundary layers, pore wetting, surface fouling, loss of enzy me activity, long-term operation, and scale-up [24]. Ravanchi et al. [21] review membra ne separation in great detail and lists the advantages and disadvantag es of this technology. 2.3 Sorbents Solid reactants can be used to react with CO2 to form stable compounds. The reactant can also be regenera ted to release the absorbed CO2 [11].These solid reactants range from metal oxides and carbonates such as calcium oxide, potassium carbonate, dolomite (CaMg(CO3)2), and other carbonate systems. Th e metal oxide will react with the CO2 to form a carbonate. While in the carbonate system, it will react with CO2 and water to form bicarbonate. The main disadvantage of mineral CO2 capture are the calcinations temperatures and slow reaction rate for lithium-containing sorbents, which shows the most promising sign for high temperature cap ture [25]. Of the ma ny metal oxides that exist or can be modified, each has its draw backs. For example, metal oxides such as sodium and potassium work best at low te mperatures, which requires the flue gas be cooled significantly prior to CO2 separation; or for magnesi um oxide the temperature range is 350 to 500C, which makes it a suitable material for conventional postcombustion capture; while calcium based oxides work best in temperatures greater than

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14 600C, making ideal for pre-combustion capture and is being investigated in this work [26, 27]. Researchers are also working on an amine-enriched sorbent [28]. Here the amine compounds are immobilized on a high surface area material, the amine then reacts with the CO2 to form a carbamate in a two-step process [11]. Microporous and mesoporous materials are also being researched and is discussed by Zelenak et al. [29] whose work studies the modification of these materials with amine to improve the efficiency and regeneration time. Zoelites, types of mes oporous and microporous materials, are also being loaded with amine to produce novel ad sorbents [23]. However, since these are synthetic materials the cost of preparation is high and a modi fied zeolite that would work at high temperatures is still being researched. Metal organic frameworks (MOFs) are also a new class of sorbents being researched. The hybrid material built fr om metal ions and organic compounds in geometrically organized struct ures [11]. MOFs require low energy for regeneration; they have good thermal stability; they are tolerant to contaminants and they are also low cost [11]. According to Millward et al. [30] MOFs offer the advantages of being totally reversible and flexible. Hydrotalcite sorbents are also being st udied by researchers. Hydrotalcites are a class of clay, layers of double hydroxides [31, 32]. According to Iwan et al. [25] hydrotalcites showed the most promis e in high temperature capture of CO2, possessing both a good adsorption capacity and rate.

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15 2.4 Chemical-looping Combustion Chemical looping combustion uses a so lid oxygen carrier to supply the oxygen needed for combustion. According to research ers at the University of Kentucky Center for Applied Energy Research, chemical loop ing provides two major advantages [33]: 1. A high-purity CO2 stream, which would make separation more efficient 2. The conversion efficiency would be gr eatly improved compared to that of using solvent for separation of the CO2 or oxy-combustion. Chemical looping combustion is in the early stages of process development. The major advantages of this technology is that air separation equipment is not required [11]. Research has to be focused on handling of multiple solid streams and the development of adequate oxygen carrier materials. Rubel et al. [33] discusses the oxidation/ reduction chemistry of several oxygen car riers and determined that ir on oxide powder and catalyst showed the most promise. Researchers at Ohio State University ar e working on three novel chemical looping gasification processes: Syngas Chemical Loop ing(SCL), Coal Direct Chemical looping (CDCL) process, and Calcium Looping Proce ss (CLP) [34]. The SCL process utilizes conventional coal gasification technology to produce a hydrogen stream for electricity production and a CO2 stream in two different reactors, which would lead to a system that does not require CO2 separation [34]. The CDCL pro cess on the other hand does not produce a syngas, but uses coal as the f eedstock. CDCL promises reductions in oxygen consumption and process intensification [34] A resulting solid iron and ash stream is produced and a gas stream of CO2 and H2O, again requiring no separation of the CO2 prior to sequestration. The last process CLP offers the same advantages as the SCL and

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16 the CDCL, but can be installed on a conventio nal gasification system removing the need for syngas cleanup after the shif t reaction. Ryden et al. [35, 36 ] and Chiesa et al. [11] discuss in detail chemical-l ooping combustion for natural gas with similar advantages. 2.5 Oxyfuel Combustion In this technology pure oxygen is used for combustion instead of air and it is combined with a recycle flue gas stream [37]. This will produce a 70% CO2 rich flue gas that can be easily purified [37]. According to a literature review conducted by Buhre et al. [37] research work has shown that oxyfue l combustion is a viable option to producing a sequestration rea dy stream of CO2, but when compared to preand post-combustion capture technologies it depended on the combustion unit retrofitted and the systems on the unit. Buhre et al. [37] as discusses in the literature that unit availability and reliability of an oxy combustion unit was still in que stion as no full-scale plant has been demonstrated and built. In 2009, McCauley et al. [38] presented favorable results from the Babcock & Wilcox Company (B&W), through its Power Generation Group, and Air Liquide oxy combustion demonstrations. Even though oxy combustion is a very favorable technology for CO2 sequestration there are many issues that still need to be resolved operationally and this can be very costly option as an air separation unit will be needed to produce the pure oxygen required for combustion [11].

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17 2.6 Cryogenic Separation Cryogenic separation is applied to CO2 capture by cooling and drying the flue gas to a temperature where solid CO2 is formed and can be separa ted [39]. Burt et al. [39] also points out that the capture efficiency is dependen t on the expansion pressure and temperature. Research is still continues on cryogenic technology to reduce the amount of energy required to operate the systems [11]. The flue gas is cooled from 60C to temperatures between -90 to -137C, then the pressure is changed until the CO2 is below its triple point, where it exists as a solid [40]. Tuinier et al. [ 41] discusses cryogenic CO2 capture using packed beds and the advantag es of cryogenic technology are chemicals are not required and the disadvantag e is that expensive water sepa ration is required to moved all traces from the gas.

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18 CHAPTER 3. CARBON DIOXIDE CAPTURE USING DOLOMITE 3.1 Dolomite Dolomite is a metastable mineral material composed of calcium magnesium carbonate CaMg(CO3)2. Vast amounts of the mineral are found in geologic deposits in lakes, shallow seafloor, and other sites [42] According to Warren [42] it can be formed in many ways: a primary precipitate, a diagenetic replacement, or a hydrothermal/metamorphic phase. Warren [42] also states that dolomite tend to be ferroan and its crystals are saddle-shaped. Goldsmith et al.[43] defines an idea l dolomite as one having a 1:1 molar CaCO3:MgCO3 ratio. However, the study showed that a number of natural dolomite samples were not ideal, as the CaCO3 content was in excess of the 1:1 molar ratio[43], the mole percent of the CaCO3 in the dolomite were discovered to be more along 55%. 3.2 Capture of CO2 with Carbonates In general, calcium-based oxides react with CO2 to form the metal carbonates. The carbonate can then be decomposed to form the metal oxide for continued capture of CO2. The main disadvantage of us ing calcium-based oxides is that they degrade rapidly [44]. Lee et al. [10] previously studi ed the calcium oxide in the use of CO2 capture by preparing a solid sorbent ma de of precipitated calcium carbonate impregnated unto a ceramic fabric as the starting material. Lee s newly developed method for the preparation

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19 of the sorbent showed a maximum convers ion of approximately 55-59% [8]. The following are the reactions for CO2 capture using a metal oxide: Reaction for the carbonation of cal cium oxide to calcium carbonate: CaO (s) + CO2 (g) CaCO3 (s) (3.1) Reaction for the calcination of cal cium carbonate to calcium oxide: CaCO3 (s) CaO (s) + CO2 (g) (3.2) Lee [8] goes on to state, in his dissert ation that degradation seen from his experimentation and the works of Barker a nd Borgwardt [45, 46] need to be addressed. Results from the literature review of calcium-based oxide for use in CO2 capture showed dolomite as being more promising than calcium carbonate [15, 47]. Hence, this research aimed to use Lees newly developed prepara tion of the calcium oxide sorbent and the results of the literature revi ew to produce a dolomite sorbent that had the flexibility of Lees sorbent and an improvement to the de gradation of the calci um carbonate sorbent. Dolomite and calcium carbonate are bot h abundantly available and naturally occurring resources, which reduc es the material cost of CO2 capture, if these products were utilized [48]. The litera ture states that if 50% conve rsion of calcium oxide takes place cyclically then 393 grams of CO2 per kilogram would have been captured [48]. Senthoorselvan et al. [48] states that th e temperatures at which calcium carbonate and dolomite undergo carbonation/calcination are applicable to power plant exit flue gas temperatures and well suited for CO2 capture. The sorbent has a maximum capacity that beyond which it degrades drastically, due to pore closures, and the inability of the CO2 molecules to reach inner calcium oxide mol ecules. The decomposition of the magnesium carbonate molecules at 750C is beli eved to leave micropores for the CO2 molecules to

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20 penetrate into the material and reach calcium oxide molecules. This feature of dolomite was the main reason it was considered in this study. The decomposition of magnesium carbonate, at lower temperatures than calcium carbonate, impedes its ability to participate in the CO2 capture, but allows the magnesium molecules to stabilize the particle st ructure and create void space for the CO2 molecules to pass through [48]. The cal cinations of calcium carbonate occurs at 750C and 385C for magnesium carbonate [15]. According to Gupta et al. [15] the carbonation of calcium oxide goes through a two step mechanism, a rapid heterogeneous chemical reaction and a slower second step which involves the penetration of calcium car bonate layer formed [15]. The extent of conversion for calcium oxide, according to the literature, ranges anywhere from 30% to 93% [15], while Lees sorbent showed conversion in th e 62-75% range.

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21 CHAPTER 4. SORBENT PREPARATION 4.1 Sorbent Preparation Dolomite was immobilized on alumina fa bric using the method described by Lee [10]. Dolocron 4512 (pulverized dolomite limest one) was selected as a starting material, as it was commercially available as a nanopowder. Precipitated calcium carbonate was added to the mixture to help the material to adhere to the fabric well. This acted as a surfactant and helped immobilize the dolomite to alumina. The general procedure for th e preparation of the sorbent began by turning on the small furnace and setting the temperature to 700 C. The alumina fabric was then cut into strips. The alumina strips of fabric were placed in the furnace for 15 and 30 minutes respectively to remove any moisture, coating or binders. Fifteen milliliters (15mL) of ethyl alcohol was placed into a 50mL centrif ugal tube. Half a gram (0.5g) of dolomite and 0.1g of precipitated calcium carbonate were added to the ethyl alcohol. The mixture was placed on a vortex for 3 minutes and then sonicated with a Sonic Dismembrator for 10 minutes. The fabric was then removed from the furnace and weighed. The mixture of sorbent and ethyl alcohol was applied to one side of the fabric and dried at 150C for 10 minutes. The material was removed from the furnace and the solution of mineral and alcohol was applied to the uncoated side. The sorbent material was then placed in furnace at 800 C for 12, 18, 55, 72 and 87 hours respectively. The sample was then weighed after sintering.

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22 4.2 Calcination-Carbonation Cycling Experiments The sorbents were tested in a TGA to obtain data on the carbonation/calcinations cycles. All tests were performed under isot hermal conditions at 750C using 200ml/min CO2 in nitrogen during the carbonation cycle du ring the carbonation phase. Pure nitrogen was supplied to the TGA during the calcinati ons phase. The system was programmed to operate automatically with carbonation and calcinations running fo r 20 minutes each. The data was collected by the computer data logger and stored for future use. The software recorded the change in weight of the sample as CO2 was cyclically adsorbed and released in the TGA chamber. 4.3 Material Characterization The sorbent materials were characterized by x-ray diffraction analysis to confirm the ma terial composition. The compound provided by Specialty Minerals used in this research is dolomite and sintering does remove the CO2 content of the dolomite leaving a com pound made up of the oxides.

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23 CHAPTER 5. RESULTS AND DISCUSSION 5.1 The Effect of Preparation Method on Capture The sorbent was first made with dolomite rocks purchased from Alfa Aesar (Ward Hill, MA). The rock had to be crushed and milled in the laboratory before it could be used. The crushed dolomite was then mixed with ethyl alcohol. The dolomite did not adhere to the fabric and fell off very easil y. The conversion efficiency for the sorbent prepared by this method was very low (Figure 4). Figure 4. Conversion Cycles for Alfa Aesar Dolomite 0.00 0.01 0.01 0.02 0.02 0.03 0.03 0.04 050100150200250300350ConversionTime (mins) Conversion vs Time

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24 The maximum conversion achieved with this method was 3.6% which means very little of the dolomite actually adhered to the alumina fabric. In a second approach, acetone replaced ethyl alcohol as the solv ent to determine if changing the solvent would al low the dolomite to be impregnated unto the fabric. The results indicated that the solvent had no effect on the material preparation in this case. The same results were obtained in terms of the physical appearance with low attachment of the sorbent to the ceramic material. A search for a surfactant that would not affect or change the properties of the dolomite was undertaken. Fullers earth, a clay -like material, made up of mineral oxides was chosen because it would inertly bind the dolomite to the fabric [49]. Different weights for Fullers earth were used to make the sorbent and it was usually inspected to determine the stability of th e sorbent. The init ial method was modified by adding 0.25g of Fullers earth to 0.5g of dolomite w ith ethyl alcohol as the solvent. Since the preliminary data showed prom ise for the development of the dolomite sorbent, a new source of dolomite was found. Dolocron 4512 was supplied by Specialty Minerals (Allentown, PA), the same provider of the precipitated calcium carbonate used in Lees work [8]. Dolocron 4512 is a brand of dolomite powder that had a mesh size of 325 (approximately 44 microns) and did not requir e further processing in the laboratory.

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25 Figure 5. Conversion Cycles for Dolocron 4512 and Fullers Earth Figure 5 shows an increase in the conversion of calcium oxide to calcium carbonate from the Alfa Aesar dolomite. Th e maximum conversion achieved was 35% and the average conversion of the four cycles of data was 34%. This results are similar to those obtained in th e literature [15]. Although the data was within the range for the Dolocron and Fullers earth mixture for conversion, this work looked at ways to improve the conversion and get it closer to the 62-75% conversion seen in Lees work [8]. This was achieved by investigating mixtures of the Dolocron 4512 and the precipitated calcium carbonate Lee 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 050100150200250ConversionTime (mins) Conversion vs Time

PAGE 35

26 used in his research. The sorbent was made using the materials and procedure from Section 3.2. The Dolocron-calcium carbonate sorbent was prepared adding 0.06g of the precipitated calcium carbonate. The graph belo w shows the conversion with respect to time for this sorbent preparation method. Figure 6. Dolocron 4512 and Preci pitated Calcium Carbonate Figure 6 shows a maximum conversion of 53%, which indicated that the change from Fullers earth to the prec ipitated calcium carbonate was beneficial. The precipitated calcium carbonate made the Dolocron adhere ev en better than the Fu llers earth and had 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0100200300400500600ConversionTime (mins) Conversion vs Time

29 Figure 7. Effect of Conditioning Time on Conversion The conversion at 87 hours of is doubl e that at 12 hours (Figure 8 and Figure 9). The results indicate that as conditioning time increased the conversion increased. A maximum threshold for conditioning exists, however further experimentation at higher conditioning time is needed to determine this value. 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 020406080100ConversionConditioning Time (mins) Conversion vs Conditioning Time

33 Figure 11. Precipitated Calcium Carbonate Comparison of Figures 10 and 11 shows th at dolomite capacity may be stabilizing after 10 cycles while precipitated calcium carbonates capacity continues to decrease even after 19 cycles. However, in both cases the number of cycles is too few to make a conclusion. 5.4 Regeneration Time The regeneration time was investigated in three experiments. The first experiment used the 10 minute calcinations regeneration ti me from Lees dissertation [15]. However, this proved to be an inadequate amount of time. The time was then increased to 15 minutes and this was still not enough time. Th e experiments were then performed at 20 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0200400600800ConversionTime (mins) Conversion vs Time

36 stability of dolomite can be clearly seen from Figure 13. Although, the dolomite adsorbs less CO2 than the calcium carbonate sorbent it s howed promise of being more stable, although longer cyclical experiment s would be needed to be sure.

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37 CHAPTER 6. CONCLUSION AND RECOMMENDATIONS 6.1 Conclusion This aim of this research was to devel op an inexpensive and efficient sorbent to capture CO2 and the design a reactor that uses the sorbent to capture CO2 emissions from power plants. The sorbent was developed using an earlier developed method to impregnate dolomite unto alumina fabric. This sorbent was successfully used to capture CO2. However, this research shows that the capacity of dolomite is about the same as calcium carbonate, although additional multi-cy cle experiments are needed to draw a conclusion. This work was also able to quantify th e capture conversion, the regeneration time, and cyclic performance of the sorbent, although for a small number of cycles. The dolomite sorbent effectively captured CO2 and its capture capacity degraded initially, but became stable at 34% after 10 cycles. Based on the experimental results in the cyclic reactions, the dolomite immobilized on the fibrous alumina fabr ic had continuous conversion in the carbonation/calcination cycles and was compar able to that of calcium carbonate.

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38 6.2 Recommendations The dolomite sorbent showed contin uous high reactivity in the cyclic carbonation/calcination cycle at 750C, however further experimentation is required at different operating a nd conditioning times, CO2 flow rates, weights of dolomite immobilized on the alumina fabric, and diffe rent fabrics, for example yttria. With additional experimentation and data, reactor design can be completed.

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Latchman, Drupatie.
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Carbon dioxide capture from fossil fuel power plants using dolomite
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ABSTRACT: The main objective of this research is to develop a simple and cost effective separation method that captures carbon dioxide from power plant flue gas, as a pure stream that can be stored using regenerable dolomite (calcium magnesium carbonate) as the sorbent. The developed dolomite sorbent was evaluated for carbon dioxide capture capacity using muti-cycle tests of cyclical carbonation/calcination experiments in the thermogravimetric analyzer (TGA) model SDT 600. The variables controlled in the experiment were weight of calcium oxide and sintering time of the sample. The dolomite materials investigated were from two sources Alfa Aesar and Specialty Minerals. The prepared sorbent, after conditioning, is in the oxide form and can adsorb CO2 to form the carbonate and be regenerated back to the oxide. The results showed that the dolomite sorbent developed can be used for reversible CO2 capture. The data from 8 multi-cycle TGA experiments show that the reversible capacity reduced in the first few cycles; however it stabilized to an average value of 34 percent after an average of 10 cycles and an average conditioning time of 15 hours. Data from two multi-cycle TGA experiments show that the dolomite sorbent is capable of an average stabilized conversion of 65% in an average of 13 cycles at a conditioning time of 87 hours.
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